A Seal has been placed on this door by Phara Dar. Perhaps there is a way to remove it.
So it would seem you have to clear the mini's just to get the door open.
On DSK don't worry.
DSK1 yes you have to kill the first two bosses in the first section in order to do the third because of mechanics. After you are flagged though you do not have to do the third ever again if you don't want to. The first one you will be doing over and over and over because Band of the Gargoyle.
DSK2 once flagged and have door keys it is just tank and spank in any order.
DSK3 - no trash mobs only bosses. They must be done in order just because of zone mechanics. Once you get them down though they are a piece of cake.